1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling time offsets between communication device and transmission points and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple UEs, and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
In detail, the CoMP is configured to a UE and multiple transmission points, the UE may communicate with the transmission points simultaneously, i.e., access a service via all or part of the transmission points. For example, a transmission point can be an eNB, a relay node or a remote antenna of an eNB (e.g., remote radio head (RRH)). More specifically, an eNB may manage only one transmission point, or may manage multiple transmission points. That is, Cell IDs of different transmission points may be different (e.g., when being managed by different eNBs), or may be the same (e.g., when being managed by the same eNB). Thus, signals transmitted between the UE and the transmission points can be easily recovered due to better quality of the signals.
Besides, when the transmission points are involved in the CoMP, one of the transmission points is a serving point (i.e., serving cell). In general, link quality between the serving point and the UE is better than link qualities between other transmission points and the UE. Further, the CoMP can be classified into two main categories: Joint Processing (JP) and Coordinated Scheduling/Beamforming (CS/CB). A main difference between the JP and the CS/CB is that data of the UE is available at all the transmission points when the JP is configured (i.e. enabled), while the data of the UE is only available at the serving point when the CS/CB is configured. The JP can be further classified into two categories: joint transmission (JT) and dynamic point selection. When the joint transmission is configured, the data of the UE can be transmitted from multiple transmission points (e.g., coherently or noncoherently) to the UE to improve signal quality and/or cancel interferences. When the dynamic point selection is configured, the data of the UE is transmitted from only one of the transmission points (e.g., according to a choice or suggestion of the UE) to the UE to improve the signal quality and/or avoid the interferences. On the other hand, when the CS/CB is configured, the data of the UE is only transmitted from the serving point to the UE, while other transmission points may adjust scheduling (e.g., stop their transmissions), or adjust beamforming (e.g., move their beams) to mitigate the interferences caused to the UE.
However, a time delay between the UE and a transmission point leads to a frequency selectivity and causes mismatch to a preferred matrix index (PMI) (i.e., precoding matrix indicator), so as to degrade throughput of the UE. In general, the time delay may be introduced by channel propagation delay (e.g., a number of microseconds) due to scattering and a distance between the UE and the transmission point. Alternatively, the time delay may also come from a time offset between the UE and the transmission point. The time delay caused by time offset results in considerable frequency selectivity, i.e., a reduced coherence bandwidth. Feeding back the PMI for each 4 resource blocks (RBs) is not sufficient according to the prior art, since channel frequency response (CFR) may vary within a resource block due to the time offset. Thus, compensating the time offset becomes an important topic to be discussed and addressed.